1. Field of the Invention
The present invention relates to a flux cored wire for duplex stainless steel, and more particularly, to a flux cored wire for duplex stainless steel and a method of manufacturing the same capable of obtaining corrosion-resistant materials which are used in seawater freshening equipment, oil refining equipment, petrochemical heat exchangers, and various chemical industrial equipment where high strength and excellent pitting corrosion resistance are required, and weld metal having excellent pitting corrosion resistance, intragranular corrosion resistance, cracking resistance, and high strength in welding structure materials of buildings or vehicles. Further, the flux cored wire can enhance welding performance and productivity owing to excellent drawability.
2. Description of the Related Art
Duplex stainless steel has excellent corrosion resistance, mechanical properties, and favorable welding performance, due to a characteristic of a minute structure thereof in which austenite and ferrite are composed at the ratio of 50:50. In early versions of duplex stainless steel, the amount of ferrite was 75-80%. This led to poor welding performance and intragranular corrosion resistance. Since the 1960s, as amounts of Cr and Ni were adjusted to maintain a compositional ratio of ferrite and austenite at 50:50, welding performance and intragranular corrosion resistance have been improved. In an actual weld zone, an amount of ferrite typically rapidly increases, thereby reducing mechanical properties and corrosion resistance of the weld zone. After that, duplex stainless steel including nitrogen (N), which is often referred to as third-generation duplex stainless steel, was developed. N is a very important component in a weld zone of duplex stainless steel and serves to help ferrite transform into austenite during cooling after welding. Accordingly, duplex stainless steel including N has better resistance to stress corrosion cracking, pitting corrosion, and intragranular corrosion than existing 300-series austenite stainless steel. In particular, the duplex stainless steel including N has a higher strength (as much as 50% higher) than existing austenite stainless steel, and a critical pitting temperature of the duplex stainless steel is higher (as much as 10 degrees Celsius or more) than existing SUS 316L stainless steel.
Examples of representative stainless steels including N include SUS304N2, SUS304LN, SUS316LN, SUS317LN, SUS329J3L, SUS329J4L, UNS S31803, UNS S32520, and UNS S32550. Stainless steel including N can be classified as austenitic stainless steel having a large amount of N and duplex stainless steel. Between them, the duplex stainless steel having high strength and excellent pitting corrosion resistance is generally used as a corrosion-resistant material in seawater freshening equipment, oil refining equipment, petrochemical heat exchangers, and various chemical industrial equipment. It is also used as a structure material of buildings or vehicles because of its high strength.
Weld materials used for welding in the above-described fields are required to have the same or more excellent physical properties than base metal. Furthermore, since favorable welding performance is required, MAG welding is typically required, and is performed using a flux cored wire with high efficiency and excellent welding performance among weld materials.
When duplex stainless steels are manufactured, heat treatment is performed after rolling. Therefore, it is easy to form a stabilized minute structure at a normal temperature. In a case of weld metal, however, it is not easy to control a change in minute structure, which occurs while the weld metal fused by welding is solidified by inherent cooling. Therefore, pitting corrosion resistance or toughness of the weld metal is more unstable than those characteristics of the duplex stainless steel. In a welding method of flux cored wire, since a heat input amount is high during welding, it is not easy to secure a relatively favorable weld zone, compared with gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), and shielded metal arc welding (SMAW).
Moreover, when the flux cored wire is manufactured, a flux is filled into a sheath of stainless steel. Therefore, there is a limitation of an amount of alloy component which can be filled. In particular, most fluxes to be used for manufacturing the flux cored wire for duplex stainless steel are expensive. Additionally, as the amount of flux alloy component to be filled increases, the degree of work hardening increases. Therefore, cutting of a wire occurs during drawing, thereby causing a reduction in productivity, accompanied by an increase in cost of weld materials.
In research relating to this, amounts of alloy components such as chromium (Cr), nickel (Ni), molybdenum (Mo), and nitrogen (N) within a weld material are adjusted to secure pitting corrosion resistance, intragranular corrosion resistance, cracking resistance, and high strength of the weld metal and a weld zone, as well as favorable welding performance. In particular, it is known that the above-described alloy components can improve pitting corrosion resistance of the weld metal.
The effects of adding the above-described components can be changed according to a variation in welding heat input which is generated in a welding spot. Therefore, through only controlling the components, it is not sufficient to enhance pitting corrosion resistance of the weld metal. Also, an increase in the amount of the components added degrades mechanical performance of the weld metal or welding performance accompanied by a variation in welding heat input. Such an increase reduces drawability during manufacturing.